Many of the world's remaining gold deposits are considered to be refractory or double refractory in nature. Refractory ores are those in which the recoveries of gold by conventional cyanidation are typically noneconomic. Low gold recoveries can be caused by naturally occurring preg-robbing carbonaceous materials, which can pre-empt the absorption of gold by activated carbon in gold recovery processes. In addition, many gold ores are also sulfide refractory. The gold in sulfide refractory ores is inaccessible to gold lixiviants because the gold occurs as; particles finely disseminated within sulfide mineral crystals or as a solid solution in the sulphide matrix. The cost of size reduction associated with liberating this gold is often prohibitive, and, in the case of gold occurring as a solid solution, the size reduction is also generally ineffective. This problem has been overcome by oxidizing the sulfides contained in the ore, thereby liberating the gold from the sulfide matrix and rendering it amenable to leaching by cyanide or other lixiviants. Methods of oxidation employ bio-oxidation, roasting, atmospheric leaching and pressure oxidation. Pressure oxidation can be performed under alkaline conditions, as in the process disclosed by U.S. Pat. No. 4,552,589, or acid conditions, as disclosed by U.S. Pat. No. 5,071,477.
In alkaline pressure oxidation, the sulfuric acid produced during the oxidation step reacts with the carbonate or other acid consumers contained in the autoclave feed according to the following reactions (based on pyrite oxidation):
1 Oxidation: 4FeS2+15O2+2H2O→2Fe2O3+8H2SO4 
2 Neutralization: CaCO3+H2SO4→CaSO4+H2O+CO2↑
The amount of carbonate, or other acid consumers, is in stoichiometric excess to the amount of acid that is generated by oxidation, and therefore the pH of the autoclave discharge is near neutral to alkaline depending upon the amount and type of acid consumers. When a portion of the gold occurs within the unreacted carbonates in the oxidized ore, the gold can remain inaccessible to lixiviants in subsequent gold leaching operations and thereby be unrecovered.
In acid pressure oxidation, the following reactions can occur:
3 Oxidation: 4FeS2+15O2+2H2O→2Fe2O3+8H2SO4 
4 Oxidation: 4FeS2+6O2+2H2O→2Fe2O3+6S°+2H2SO4 
The prevalence of one reaction over the other depends on a number of factors including operating temperature, pressure, molecular oxygen overpressure, and residence time. No matter which of the oxidation reactions prevails, a substantial, amount of sulfuric acid is produced and must be neutralized. Acid neutralization costs can significantly increase plant operating costs.